1. Field of the Invention
This invention relates to voice operated switches and more particularly to voice operated switches for controlling transmit-receive modes of loudspeaking telephones.
2. Description of the Prior Art
In many situations where it is desirable to use voice operated switches, ambient noise conditions preclude or hamper the use of such switches. These situations include the use of voice activated machinery in workshops, near printing presses, in typewriting rooms, and the like, and especially the use of voice operated switches to control the transmit-receive modes of loudspeaking telephones, or mobile telephones in automobiles, trains, or ships. One particular problem is that certain noises such as, for example, automobile engine noise during sudden acceleration, or automobile chassis noise when driving over potholes, have sound pressure level characteristics which resemble human RMS speech signals.
One prior solution to the problem was to place the speech microphone very close to the mouth. This improved voice intelligibility despite high ambient noise levels, but seriously restricted the speaker's freedom of movement. At sufficiently high ambient noise levels, this arrangement completely failed to distinguish voice from noise levels.
Another previous solution was the use of a so-called noise microphone placed some distance from the speech microphone. Only the signal resulting from subtracting the noise level from the voice level was used. This system performed well only under ideal conditions, i.e., in environments free from acoustic reflections. Where acoustic reflections were present, the voice level often appeared on the noise level, and the subtraction eliminated the voice signal altogether.
Yet another approach was to rectify the signal from the microphone and compare the minimum and maximum levels with the minimum and maximum levels of the receiving party's signals from the receiver. This technique satisfactorily eliminated the effects of high level background noise, but failed to adequately distinguish voice from noise when the noise levels fluctuated in a manner resembling the RMS component of speech.
These and other prior solutions are shown, for example, in Bertholon U.K. Patent Application No. GB2,003,002 A, filed Feb. 28, 1974, for Detecting Speech In The Presence Of Noise, in which a speech detector circuit closes a transmission switch when the energy content of a sound burst measured over a period not exceeding 100 ms exceeds the ambient noise level by more than a predetermined threshold. This circuit does not adequately distinguish between voice sound bursts and noise sound bursts resembling the RMS component of speech signals.
Breeden, U.S. Pat. No. 3,751,602, issued Aug. 7, 1973, shows a control circuit to achieve complementary switched gain in the transmit and receive channels of a loudspeaking telephone. Only one microphone is employed, however, and even with optimal selection of the noise rectifier and time constant circuits, the control circuit still does not adequately distinguish between voice and RMS resembling noise levels.